This invention relates to the extraction of uranium from its ores, and in particular to an improved method for acid leaching of uranium from its ores and especially from those ores which contain uranium as a finely-disseminated refractory material, such as brannerite or uraninite, as found for example in the Elliot Lake area of Canada.
Most current uranium ore processing employs leaching in dilute sulphuric acid (less than 1N), although an alkaline leaching process has been developed for ores containing a high content of acid-consuming constituents such as carbonates. The established acid leaching process for Elliot Lake ores is somewhat more severe than the above, requiring fine grinding (40 to 60 percent-200 mesh), a long reaction time (2-3 days) in an aerated pulp of relatively high density (60-70% solids), a temperature of 60.degree. to 80.degree. C., and an acid addition of 60-80 lb H.sub.2 SO.sub.4 per short ton of ore.
Approximately half of the added acid is rapidly consumed in reactions with acid-consuming constituents of the ore, while the remainder of the acid provides a free acid concentration of about 0.7N H.sub.2 SO.sub.4 in order to achieve a practical leaching rate. At the completion of the leach the excess free acid must be neutralized before further processing for uranium recovery. As this leaching process requires long residence times at fairly high free acid concentrations, tanks in which it is carried out have to be large and rubber-lined and thus represent a major capital cost item. Mechanical agitation of the leaching tanks has proved to be unsatisfactory and therefore air agitated pachuca tanks are used. Large quantities of compressed air are consumed in these tanks, and associated evaporative heat losses are very high, leading to significantly higher operating costs than for mills handling less refractory materials.
Canadian Patent Specification No. 938,453, issued Dec. 18, 1973, describes and claims a strong acid pug leaching process for such ores, using sulphuric acid of a concentration greater than 4N at a typical addition of 60 lbs H.sub.2 SO.sub.4 per short ton of ore, and a curing temperature of 65.degree.-100.degree. C. This process is operated on relatively coarsely-ground ore of a top size not substantially smaller than 1 mm. However, the use of strong acid at elevated temperatures has certain disadvantages, particularly:
Extraction of unwanted impurities tends to be higher than with dilute acid; PA1 In order to adequately wet the ore with more concentrated acid, more contained H.sub.2 SO.sub.4 must be added than is otherwise required for satisfactory leaching; PA1 Attack on most commonly used corrosion-resistant materials is more severe than with dilute acid, especially above 75.degree. C., where unusual materials must be resorted to; PA1 Dry grinding must be employed if a relatively high acid concentrations is required, which is poor practice from an environmental viewpoint for uranium ores.
The limit of conventional fine crushing is presently 3/8 inch (9.51 mm), and hence to comminute the ore to a top size of about 1-1.5 mm for the above mentioned strong acid process, dry grinding must be employed. Dust control in dry grinding circuits is difficult, and in the case of the Elliot Lake conglomerate ores in particular, the dust hazard is extreme due to the radioactivity and high free silica content of these materials.
I have now found that an increase in the acid concentration does not necessarily equate with higher uranium extraction and faster leaching rates, and that maintenance of a suitable oxidizing environment is more important than acid concentration for obtaining high uranium extraction from refractory ores, maximum extraction being of prime importance for a product of such high value as uranium.